Wednesday, 23 July 2014

Running backstays

The inner forestay, when tensioned, will bend the mast and sag, unless the mast is supported from the opposite side. This is usually achieved with running backstays (runners) - a pair of wires attaching to the mast near the inner (or, on fractional rigs, only) forestay and tensioned alternately, with the windward one taking the load while the leeward one is eased. The running backstays are needed to fly the staysail on the inner stay, just like the main backstay must be tensioned to fly the jib on the main forestay.
Running backstay (right) supporting the mast
The loads on running backstays are comparable to those on other standind rigging - on our boat, all shrouds and the inner forestay are made from 6mm 1x19 stainless wire rope, with a breaking strength of ~3000kg. For the runners, we choose 5mm braided dyneema - at a similar breaking strength of 2600kg, it weighs a fraction of an equivalent steel wire, and the end terminals are easy to install by splicing, making it also cheaper than the equivalent wire rope assembly with swaged terminals. The only downside of dyneema is the lower life expectancy - but the often quoted UV degradation "in one season" is a thing of the past with modern UV-stabilized braids.

Hollow (single-)braid lines are very easy to splice using little more than a benbt piece of wire, and many splicing instructions can be found on the internet. Attention to detail will result in a professional-looking (and very strong!) line. I will be happy to elaborate on any of the steps - ask away.

First, I spliced the eyes for the upper terminals. After cow-hitching the line to the steel ring, I marked out the splice and tapered the running ends.
Measuring, marking and tapering the ends

Next, I created a locked Brummel eye to avoid any chance of the splice pulling out while not tensioned.
Creating a locked Brummel eye

The tail was than buried - a splice length of 30+cm, with the end gradually tapered, should provide an eye stronger than the rope itself.
Tightening the eye and burying the end

Finished eye splice hitched to T-terminals

After test-fitting the T-terminals, we cut the lower end of the runners to length. I also marked the area of each runner that could chafe on the spreaders or shrouds, and added a sheath in those areas by feeding the dyneema through the the sheath (in at 1/3, out at 2/3), and burying the tails of the sheath back into the dyneema braid.
I then spliced eyes for the low-friction rings at the bottom end of each runner (lighter and stronger than blocks).
Low-friction rings and chafe protection sheath spliced into the dyneema

The T-terminals were then taken up the mast one last time, and locked in place using riveted steel straps to stop them escaping.

Next, we needed strong points on deck to tension the runners. We installed two M8 eye bolts through the toerail at the aft end of the spinnaker track - with a 150x50x3mm stainless backing plate below - on each side. A line was attached to one eye, passed through the flying alloy ring for a 2:1 purchase (minus friction), and through a block attached to the second deck eye to a winch. While the eye bolt's quoted strength seemed plenty, the extra load from the end of the line taken to a winch was enough to deform the eye!
Outcome of strength testing
While I doubt it would have ever failed catastrophically, we thought it prudent to replace the bolts with stronger Wichard-made ones (the eye part of these is smaller and thicker, making it more suitable for side loads), and beef up one of each pair to M10. New strongest part of the boat? Maybe. If I was to do it again, I would go for U-bolts, although finding one long enough to pass through our toerail (80+mm) is a little tricky.

Next, we considered various options for tensioning the inner forestay. A system similar to that used on the backstays seemed simplest, and we had a few meters of dyneema going spare. Some rings spliced to the lower terminal, a ring spliced to the deck bolt via an endless strop, and a low-stretch control line lead back to a coachroof winch via one of the newly replaced clutches completed the system.
Endless double dyneema eye

Tensioner line lead aft...

Winch and clutch for stay tensioning
When not in use, the inner forestay and the runnging backstays can be tied to shroud bases, and with the forestay tensioner and the staysail halyard both lead aft, the staysail can be hoisted or dropped from the cockpit. The forestay tensioner will also double as a spinnaker pole downhaul in lighter winds.

Everything seems to work well, and we have the option of adding more forestay tension by increasing the purchase if it turns out to be insufficient in strong winds. A block with a sufficient breaking strength looks massively disproportionate on our boat, while friction rings are strong, cheap, light and easy to attach with dyneema strops. Same goes for dyneema runners vs conventional wire - a lot of the "racing" tec can be useful on a cruising yacht.

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